how to manage big project?
Presumably the leaves will be part of a plant - in which case you'll want to set them to be Children of the parent plant. This way when you manipulate the Parent (eg, move, rotate, scale) the children will be similarly affected rather than being left behind. You can easily Parent objects by selecting all the children, holding Shift and selecting the Parent (so the parent is the last object selected) and then pressing Ctrl+P and select from the Parent menu. When you do this the outliner will show them as being within the parent object which can be collapsed to hide them (ie, the Parent effectively acts as a folder for its children).